1. Field of Art
The present invention relates to a solid-liquid separator for separating impurities to be collected from raw water.
2. Description of Relevant Art
Water treatment employs in a process thereof a solid-liquid separation treatment such as a gravitational settling, flocculation sedimentation, or dissolved air flotation.
In the gravitational settling or flocculation sedimentation, raw water inflows to a settling tank, where impurities contained in raw water as targets of collection heavier in specific gravity than water are settled by use of differences in specific gravity between water and impurities, and a supernatant is taken as treated water, whereby raw water is separated into impurities and treated water. In this case, the settling rate is varied in accordance with impurities' specific gravity, size, etc. For instance, for impurities relatively small in settling rate, the settling rate is raised by increase in volume of the settling tank, or the settling efficiency is raised by use of an inclined plate or inclined pipes for enhancing the settling rate. However, even with such a rise in settling efficiency by use of an inclined plate or inclined pipes, there is an issue of the residence time still requiring one hour or more, because of the limit in reduction of residence time, as well as the size in volume of settling tank.
In the dissolved air flotation, for buoyant impurities such as fat or solid materials relatively light in specific gravity, air is pressured to dissolve in recirculating separated water or the like, which is let to inflow to a separation tank, where microscopic bubbles are formed and attached to impurities to surface for separation, whereby raw water is separated into impurities and treated water. For the dissolved air flotation, impurities with adherent bubbles have a surfacing speed of 200 mm/min or less. Therefore, dissolved air flotation also needs a long time for treatment, as an issue.
For reduction of the treatment time having been a problem in gravitational settling or dissolved air flotation in the past, there is a method disclosed in Japanese Patent Application Laid-Open Publication No. 11-333320, in which raw water is swirled in a container to separate impurities by use of centrifugal forces.
In the method of swirling raw water as disclosed in Japanese Patent Application Laid-Open Publication No. 11-333320, swirling streams should have high speeds to produce strong centrifugal forces, and impurities once separated are caused to roll up by high speeds, with a potential re-making to treated water, as an issue.
For prevention of the re-mixing of impurities, there is a technique disclosed in Japanese Utility Model Registration Application Laid-Open Publication No. 5-9656, which produces swirling streams in a container of a double-cylinder structure with an inner cylinder made of a porous material or as a filter.
There is also a technique disclosed in Japanese Patent Application Laid-Open Publication No. 2002-66387, which includes a container for producing swirling streams, and has a nozzle provided to a lower portion of the container for discharging impurities, and adapted to function as a check valve made of an elastic material.
However, the provision of a doubled container is unable to cope with a potential re-mixing of impurities due to a roll-up in a central region of the container. The provision of an elastic check valve constitutes, if the elasticity is too high, a difficulty for impurities to pass through, resulting in an insufficient collection of impurities, and if the elasticity is too low, a marred function of check valve, resulting in a damage on check valve while running.
The present invention has been devised in view of such issues, and it is an object of the present invention to provide a liquid-solid separator adapted to prevent impurities separated from raw water from being re-mixed in raw water, allowing for an enhanced separation performance.